Osteoporosis (porous bone disease) is a disease of the skeleton that can have debilitating effects on many US veterans. An estimated 44 million Americans, or 55 percent of the people 50 years of age and older, are currently at risk for osteoporotic fracture. Improved treatment options for the disease require a greater understanding of the cellular events and signaling pathways that control bone metabolism. The proposed research capitalizes on human diseases that result in very high bone mass. The genetic causes of these high bone mass diseases?craniotubular hyperostosis, sclerosteosis, van Buchem's disease?provide insight into how bone mass can be manipulated in osteoporotic patients to improve their skeletal health and prevent fractures. Many of the high-bone-mass associated diseases are caused by mutations in a cell signaling pathway called ?Wnt.? Thus, manipulation of the Wnt pathway holds great promise for skeletal health improvement. This pathway is particularly attractive as a therapeutic target because it can be manipulated to increase new bone formation, rather than simply prevent further bone loss (which is how all but one of the currently available FDA-approved therapies work). The long term goals of the proposed project are twofold: first, we seek to understand how the secreted inhibitors of Wnt signaling function as a coordinated unit (i.e., a milieu), by adjusting their expression levels when other members of the unit are adjusted (e.g., inhibited or deleted). Those adjustments in expression in the members of the milieu represent prime targeting opportunities to enact large changes in anabolic action in bone, as our supporting data suggest. We also seek to understand how this Wnt inhibitor milieu controls the anabolic action of mechanical loading?a potent anabolic stimulus that has lasting benefits to the skeleton. We seek to understand whether certain members of the inhibitory milieu function as ?homing signals? to ensure that new bone is added where it is needed most ? to the high strain regions of the bone, and that it is not added where it is not needed ? to the low strain regions of the bone. Again, our data suggest that the Wnt inhibitory milieu plays a significant role in this process. Our second goal of the application is to conduct functional studies targeting the Wnt inhibitor milieu, that have direct applicability to future therapeutic approaches in patients. Bone wasting conditions such as mechanical disuse (e.g., bedrest, paralysis) and glucocorticoid therapy (a drug used for treating inflammation and immuno- suppression) are common among veterans. Based on measurements we and others have made regarding the changes in expression of Wnt inhibitors following disuse and glucocorticoid exposure, we hypothesize that the ?compensatory milieu? of four Wnt inhibitors?Sost, Dkk1, sFrp4, and Wise?coordinate via unknown mechanisms to prevent anabolic action in the presence of disuse glucocorticoid therapy. We propose to target the entire milieu in different combinations, to determine whether we can restore anabolic activity in mice exposed to these bone wasting conditions. If so, those studies would have far-reaching implications for the design of therapies aimed at treating veterans with disuse- and glucocorticoid-induced bone deficiencies. Another functional study we will undertake, which also capitalizes on the biology of the Wnt inhibitor milieu, is to determine whether we can reduce the dose/volume of Sost antibody required to generate a significant anabolic response by additionally blocking accessory Wnt inhibitors that are part of the compensatory milieu. We have already shown that we can dramatically increase the anabolic efficacy of Dkk1 antibody if we use it in the presence of Sost inhibition. We anticipate a significant osteoanabolic effect using much lower doses of antibody if we simultaneously block other accessory Wnt inhibitors. In this renewal Merit application, we address these questions in order to identify new ways to improve bone health among the veteran population, and among the public in general.